Secondary lithium ion cell or battery and protecting circuit electronic device and charging device of the same

ABSTRACT

The present invention provides a new method for improving capacity, average operating voltage and specific energy of a secondary lithium ion cell or battery. This method is achieved by means of properly adjusting the ratio between a positive material and negative material, which is calculated by theoretical specific energy, and properly increasing charge cut-off voltage. The present method can greatly increasing specific energy and average operating voltage of a secondary lithium ion cell without influence on recycle property of the cell. The present invention also provides a secondary lithium ion cell or battery practicing the method, a protecting circuit adapted for the secondary lithium ion cell or battery, a electronic device using said protecting circuit and said secondary lithium ion cell or battery, and a charging device for the secondary lithium ion cell or battery.

TECHNICAL FIELD

[0001] The present invention relates to a new method for improvingcapacity, average operating voltage and specific energy of a secondarylithium ion cell or battery, and to a secondary lithium ion cell orbattery prepared by using the method, a protecting circuit adapted forthe secondary lithium ion cell or battery, a electronic device using thesecondary lithium ion cell or battery, and a charging device for thesecondary lithium ion cell or battery.

BACKGROUND ART

[0002] The industry of lithium ion cell develops quickly since SONYcorporation of Japan invented and commercialized a secondary lithium ioncell. Up to 2000, the manufactures of lithium ion battery around theworld compete allsidedly for improving the competitive power of theirproducts mainly around the key issue, the capacity of lithium ionbattery. At present, the improvement of capacity of commercializedsecondary lithium ion battery generally depends on the increase ofloading quantities of active substances (positive electrode materialsand negative electrode materials). However, the limitation of the volumeof lithium ion battery greatly restricts the increase of the batterycapacity. For notably raising the capacity, the researches for thedevelopment of active substances (positive electrode materials andnegative electrode materials) having higher specific energy are beenconducting around the world, but so far, there is no notablebreakthrough in this aspect for various technical difficulties.

[0003] In fact, the positive electrode materials and negative electrodematerials used in the current secondary lithium ion battery haverelatively higher theoretical capacity, and the problem merely lies inthe lower actual utilization rate of said capacity. For example, lithiumcobalt oxides as a positive electrode material of secondary lithium ioncell has a theoretical capacity of 248 mAh/g, while the actually usedcapacity of it is merely about 140 mAh/g, i.e., about half of saidtheoretic capacity is not utilized. This is mainly caused by thelimitation of charge cut-off voltage commonly used in the art. Atpresent, the charge cut-off voltage of single secondary lithium ion cellis limited to no more than 4.2 V, and this is well accepted as atechnical requirement in the industry of manufacture of secondarylithium ion battery. Further, all lithium ion batteries in the marketsaround the world are manufactured under this technical requirement Forexample, the charge cut-off voltage is limited to below 4.2 V and theovercharge release voltage of its protection circuit is controlled below4.15 V during the formation of single lithium ion cell. The reasons thatthe charge cut-off voltage being limited to below 4.2 V lie in thefollowing opinions in the prior research results and documents: althoughthe capacity and average operating voltage are improved by increasingthe charge cut-off voltage, the positive electrode materials and thenegative electrode materials will undergo structure change, theelectrolyte may decompose, and the recycle property of the cell will beadversely affected when the charge cut-off voltage is greater than 4.2V.

[0004] For instance, as to lithium cobalt oxides that is used aspositive electrode material in the most commercial lithium ionbatteries, the charge cut-off voltage is limited to below 4.2 V and theactual capacity is 120-140 mAh/g, i.e., about 50% of the theoreticalcapacity, although many documents indicate that the charge cut-offvoltage can be over 4.2 V for a test cell using metallic lithium ascounter electrode. In fact, according to MIZUSHIMAK et al., “A newcathode material for batteries of high energy density”, Mater. Res.Bull., 1980, 15:783, the quantity of dedoped lithium ion increases withthe increase of charge voltage, and the electrochemical capacity oflithium cobalt oxides increases accordingly. However, the study deemsthat the reversible charge-discharge voltage is about 4.3 V whenmetallic lithium is used as counter electrode, and when said voltage ishigher than 4.3 V, the structure of lithium cobalt oxides changes andthe lattice parameter C decreases from 4.4 nm to 4.0 nm, and thus therecycle life of cell is affected.

[0005] G. PISTOIA et al., J. Power Source, 56(1995), 37-43, deems thatthe structure the lithium cobalt oxides changes with the charge voltage,and the coexistence of monoclinic phase and hexagonal phase will appearwhen the charge voltage is over a certain value, which will spoil therecycle property of cell. The results of experiments showed that as to atest button cell having metallic lithium as negative electrode, thecapacity of lithium cobalt oxides reaches 159 mAh/g when the chargecut-off voltage is 4.35 V, but it drops to 135 mAh/g after severalcycles; and the capacity attenuates quickly when the charge cut-offvoltage is 4.25 V. This document takes the opinion that lithium cobaltoxides maintains excellent recycle property and a capacity about 130mAh/g only when the charge cut-off voltage is 4.15 V, and thecorresponding voltages of monoclinic phase and hexagonal phaseseparately is 4.05 V and 4.17 V, i.e., both of them are below 4.2 V.

[0006] In addition, Lei Yongquan, “Materials for New Energy” (inChinese), 2000, p136, discloses that the decomposition voltage ofelectrolyte solution using LiPF₆ as electrolyte and EC/DMC as mixturesolvent is 4.2 V, and thus deems that the electrolyte solution will bedecomposed and the recycle life will be affected when the charge cut-offvoltage is above 4.2 V.

[0007] In 1990, Sony Corporation issued the lithium ion cell using cokeas negative electrode, which has a charge cut-off voltage of not morethan 4.20 V, and it is accepted as a common technical requirement oflithium ion cells thereafter.

[0008] The prior art deems:

[0009] 1. The increase of charge cut-off voltage will change thestructure of positive electrode material, which mainly exhibits at thefollowing two aspects: one aspect is that the phase change, i.e., thecoexistence of monodinic phase and hexagonal phase and the conversionbetween them may seriously affect the recycle life of lithium ion cell;and another aspect is that the change of lattice parameter may narrowthe channel for passing lithium ions, squeeze the space occupied bylithium ions, jam the channel of lithium ions, and decrease the recycleproperty of lithium ion cell.

[0010] 2. The elevated charge cut-off voltage may decompose theelectrolyte solution, and the loss of electrolyte solution renders thetransportation of lithium ion more difficult, and thus the recycle lifeof cell is seriously affected.

[0011] Therefore, it can be seen that the limitation of charge voltagerestricts the actual utilization of active electrode materials. Underthis condition, even if new positive electrode material and negativeelectrode material having higher specific energy are developed, thelithium ion cell cannot exhibit the best performance. Hence, it isurgently needed to provide a method that can improve the efficacy ofactive substances of lithium ion cell, consequently increase thecapacity and average operating voltage, and maintain the better cellperformance simultaneously.

SUMMARY OF INVENTION

[0012] As to the common opinion in the art that increasing the chargecut-off voltage above 4.2 V may greatly shorten the recycle life, theinventor of the present invention conducted a large number ofexperiments and studies to elevate the charge cut-off voltage and thecapacity of cell. Contrary to this opinion, the inventor unexpectedlyfound that the efficacy of electrode active materials is greatlyincreased by increasing the charge cut-off voltage and propertyadjusting the ratio of positive electrode material to negative electrodematerial of single lithium ion cell. Consequently, the specific energy,capacity and average operating voltage of secondary lithium ion cell orbattery are improved, while the performance of cell is substantially notchanged. The present invention is fulfilled based on the aforesaiddiscovery.

[0013] It is one object of the present invention to provide a new methodfor improving capacity, average operating voltage and energy density ofa secondary lithium ion cell or battery, wherein the charge cut-offvoltage of the singe cell is greater than 4.2 V, and less than 5.8 V,and the ratio of positive electrode material to negative electrodematerial of the single cell is from 1:1.0 to 1:2.5, preferably from1:1.16 to 1:2.5, as calculated by the specific capacity with the chargevoltage limited to 4.2 V.

[0014] Another object of the present invention is to provide a secondarylithium ion cell or battery, wherein the single secondary lithium ioncell has an charge cut-off voltage of greater than 4.2 V but less than5.8 V, and the ratio of positive electrode material to negativeelectrode material of the single cell is from 1:1.0 to 1:2.5, preferablyfrom 1:1.15 to 1:2.5, as calculated by the specific capacity with thecharge voltage limited to 4.2 V.

[0015] Yet another object of the present invention is to provide aprotecting circuit adapted for the secondary lithium ion cell orbattery, said protecting circuit having a first overcharging protectionvoltage of greater than 4.35 V, and an overcharge release voltage ofgreater than 4.15 V.

[0016] Yet another object of the present invention is to provide anelectronic device using the secondary lithium ion cell or battery aspower supply, said electronic device comprising a protecting circuithaving a first overcharging protection voltage of greater than 4.35 V,and an overcharge release voltage of greater than 4.15 V.

[0017] Yet another object of the present invention is to provide acharging device for the secondary lithium ion cell or battery, saidcharging device controlling an charge cut-off voltage for the singlelithium ion cell within the range of greater than 4.3 V but less than5.8 V, preferably within the range from 4.3 V to 5.2 V, and morepreferably from 4.3 V to 4,8 V.

EMBODIMENTS FOR CARRYING OUT THE PRESENT INVENTION

[0018] Being contrary to the common opinion in the art that the chargecut-off voltage shall be controlled below 4.2 V, the present inventorunexpectedly found that the capacity, specific energy and averageoperating voltage of the secondary lithium ion cell or battery werenotably improved with little cost and with the other propertiessubstantially unchanged, when investigating the effect of the elevatedcharge cut-off voltage on the cell performances with a large number ofexperiments of gradually elevating the charge cut-off voltage andappropriately adjusting the ratio of the positive electrode to thenegative electrode of the single lithium ion cell. In the presentinvention, “lithium ion cell or battery” means that the presentinvention can be applied to either a single lithium ion cell, or asingle lithium ion cell comprising a protecting circuit, or a batterycomprising a number of single lithium ion cells, or a battery comprisinga number of single lithium ion cell and protecting circuits. Forbriefness, sometimes it may also be termed as “lithium ion cell”. Inaddition, “theoretical capacities of positive electrode and negativeelectrode” means the capacities of the positive electrode and thenegative electrode calculated with an charge cut-off voltage set at 4.2V.

[0019] The present inventor studied the relation between the chargecut-off voltage and the cell properties of secondary lithium ion cell bygradually elevating the charge cut-off voltage. For example, in theformation test for the commercial secondary single lithium ion cells andthe self made single secondary lithium ion cells, the inventor elevatedthe charge cut-off voltage from 4.2 V to 4.30 V, 4.35 V, 4.40 V, 4.45 Vand 4.6 V, and the results showed that when the charge cut-off voltageis, 4.3 V, 4.35 V and 4.40 V, the specific energy of cell increases by6-20% than that under the charge cut-off voltage of 4.2 V, and the cellsstill maintain excellent recycle property, e.g., the capacity maintainsmore than 95% after 50 cycles, and more than 80% after 300 cycles,.However, when the charge cut-off voltage is 4.45 V or high, the specificenergy increases by about 30%, but the cell exhibits inferior recycleproperty, e.g., the capacity maintains merely 83.9% after 6 cycles.

[0020] For investigating the reasons of poor recycle property when thecharge cut-off voltage is 4.45 V or high, the inventor adjusted theratio of positive electrode material to negative electrode material,i.e., said ratio was adjusted within the range from 1:1.3 to 1:2.5calculated according to the theoretical capacity under the chargecut-off voltage of 4.2 V, and then the single lithium ion cell havingthe adjusted ratios were tested by charging at an charge cut-off voltageof 4.45 V, 4.6 V, 4.8 V, 5.0 V, 5.2 V, 5.4 V, 5.6 V and 5.8 V,preferably said charging tests were conducted during the formation andtest of the cells. Experimental results show that the specific energywas greatly increased under the charge cut-off voltage of 4.45 V orhigher, and the corresponding recycle property was essentially notaffected, when appropriate ratios of positive electrode material tonegative electrode material were adopted.

[0021] In the present method, the elevating of the charge cut-offvoltage is advantageously carried out during the formation andexamination of the cell. The use of charge cut-off voltage of 4.20 V orhigh could increase the specific energy of cell, the capacity ofpositive electrode active material and the average operating voltage,and activate the cell so that the cell reaches its optimal working statewith simultaneously achieving the effect of the formation, substantialimprovement of the specific energy of commercial lithium ion cells, andimparting the competitiveness of lithium ion cells without altering theoriginal process.

[0022] The mechanism of the above experimental results needs moreresearches. Without limited by any theory, the presumed reasons are asfollows.

[0023] 1. After the elevation of charge cut-off voltage, the amount oflithium ion dedoped from the positive electrode greatly increases, whichrenders an unmatched state to the original negative electrode material,and deposition of excessive lithium ions on the surface of negativeelectrode as metallic lithium, which jam part of the channel for passinglithium ions and result in the reduction of capacity and thedeterioration of recycle property. The increase of the amount ofnegative electrode material meets the requirement of doping therelatively excessive lithium ions, and avoids the deposition of lithiumions on the surface of negative electrode as metallic lithium, and thusthe maintaining properties (self-discharge properties) and recycleproperty are not affected. The suitable ratio of positive electrodematerial to negative electrode material will avoid the deposition ofmetallic lithium on the surface of negative electrode and theobstruction of channel for passing lithium ions, and consequently avoidthe attenuation of capacity of cell. In particular, when the chargecut-off voltage of lithium ion cell is greater than 4.45 V, the negativeelectrode material of the ordinary commercial lithium ion cell is stillmore deficient, and the excessive lithium ions will deposit on thesurface of negative electrode and form metallic lithium, which jams thechannel for passing lithium ions and attenuates the capacity of cell.The great increase of content of negative electrode could reduce theattenuation of capacity of lithium ion cell caused by the increase ofrecycle times. According to this viewpoint, the inventor performed a lotof experiments, and the results prove the aforesaid assumption (see theExamples).

[0024] 2. Within a certain range of charge cut-off voltage, thedecomposition of little electrolyte solution brings about negligibleeffect on the recycle property of cell. The electrolyte having a highdecomposition potential or an additive increasing the decompositionpotential of electrolyte solution may bring about better performance.The decomposition of electrolyte solution mainly occurs on the positiveelectrode. Although prior documents disclosed that the decompositionvoltage of the electrolyte solution comprising LiPF₆ as electrolyte andthe mixture of EC/DMC as solvent on the surface of aluminum foil is 4.2V, according to the results of experiments, this factor essentially doesnot affect the recycle life of the currently commercialized lithium ioncell. Namely, even though the electrolyte solution decomposes under avoltage higher than 4.2 V, the electric energy is mainly converted intochemical energy and the electric energy involved in the decomposition ofelectrolyte is very little, thus, this decomposition of electrolyte canhardly affect the recycle life of the lithium ion cell. As to thevoltages above the decomposition voltage, such as above 5.0 V, thesubstance A can be added into the electrolyte solution, or theelectrolyte solution B having a higher decomposition voltage can beused. The decomposition potentials of the components of electrolytesolution commonly used in the art are depicted in Table 1. It can beseen that the lowest decomposition voltage of solvent is above 4.5 V.TABLE 1 Decomposition potentials of various mixture electrolytesolutions Solute LiClO₄ LiAsF₆ LiPF₆ Mixture Decomposition DecompositionDecomposition solvent potentials (V) potentials (V) potentials (V)PC:DME 4.51 4.72 PC:DEC 4.5 4.5 PC:EC 4.5 PC:EC:DME 4.5 4.62 PC:DEC:2MLF4.52 EC:DEC 4.9 4.8 EC:DMC 4.9 B 5.9

[0025] 3. As to the reason that the lithium ion cell does not appearattenuation of capacity caused by the change of structure as mentionedin the prior documents when the charge cut-off voltage is above 4.2 V,it may be due to that: after the ithium ions first intercalaton intoanode, there is about 10% lithium ions which form a SEI film, so thatthe actual space of the lithium ions is more than the space that shouldbe occupied by the lithium ions, i.e., the space available to thelithium ions exceeds about 10% of the space needed by the activatedlithium ions, thus, although the elevation of the charge voltage changesthe structure of positive electrode material, i.e., reduces the latticeparameter, the recycle life is not affected because the space occupiedby the actually deintercalation and intercalation lithium ions is lessthan the space actually possessed by the positive electrode material,and thus the change of structure in a certain extent will not obstructthe dedoping and doping of lithium ions and will not affect the recyclelife of cell. Using positive material more stable in structure withrespect to the change of charge voltage exhibit better performance.Although the positive electrode generates inert substance poor inconductivity when overcharged, according to the results of experiments,it occurs only when the lithium ions completely dedoped. For example, asto lithium cobalt oxides, lithium nickel oxides, and doped lithiumcobalt oxides and lithium nickel oxides, when they are charged with 3C5Acurrent, the experimental data show that only when the voltage is about6.20 V the lithium ions completely dedoped, with releasing a lot ofoxygen, and forming an insulator. As to lithium manganese oxides anddoped lithium manganese oxides, the lithium ions completely dedopedunder the same charge current when the cell potential is about 6.50 V.Generally, the specific energy of positive electrode active substanceactually used in the present commercialized lithium ion cell is far lessthan the theoretical capacity thereof, and even if the charge cut-offvoltage is elevated up to 5.8 V, the theoretical capacity cannot beachieved under the proviso that suitable formulation is used, thus, thecell is not overcharged. Therefore, the present method still enjoyssatisfactory safety.

[0026] 4. Besides the aforementioned factors, the self-discharge ofcell, the selection of current collector, and the formation of passivefilm all affect the recycle life of lithium ion cell, while thesefactors mainly depend on the preparation of lithium ion cell. It isbelievable that if the optimal processes are used, the capacity, averageoperating voltage and specific energy can be greatly improved, while therecycle life and other properties of the cell are not affected.

[0027] Hence, the present invention is to provide a novel method foreffectively improving the specific energy and average operating voltageof secondary lithium ion cell or battery. Contrary to the prior art, thepresent method elevates the charge cut-off voltage to greater than 4.2 Vbut less than 5.8 V, and control the ratio of positive electrodematerial to negative electrode material of single lithium ion cell at1:1.0 to 1:2.5 calculated by theoretical specific energy, so as toincrease the efficacy of electrode active materials, to improve thecapacity, energy density and output voltage of cell, and to maintain theperformance of cell. For achieving the better effect, the charge cutoffvoltage used in the present invention is preferably 4.3-5.2 V, and morepreferably 4.2-4.8 V. In addition, the ratio of positive electrodematerial to negative electrode material of cell is 1:1.0 to 1:2.5calculated by the theoretical capacity under the charge cut-off voltageof 4.2 V. The experiments showed that when the process parameters gobeyond the aforesaid ranges, the properties of cell are deteriorated andunsuitable for use. When the ratio of positive electrode material tonegative electrode material is less than 1.0 calculated by theoreticalcapacity, the recycle life of cell is excessively reduced, while whensaid ratio is greater than 2.5, the volume efficiency of cell is notablyreduced. Further, when the charge cut-off voltage is greater than 5.8 V,the cell has inferior properties and is unsuitable for use.

[0028] It can be seen that the capacity, specific energy and averageoperating voltage of secondary lithium ion cell can be greatly improvedby elevating the charge cut-off voltage and by appropriately adjustingthe ratio of positive electrode material to negative electrode materialcalculated by the theoretical capacity, while the recycle property ofcell is not affected, which renders the lithium ion cell possess morecommercial value and broader application range. Hence, it can also beseen that the present method further optimizes the utilization ofpositive electrode material, and thus is an economical method. Theinventors believe that the method can still be used to positive andnegative electrode materials having higher specific energy developed infuture, and achieves the optimal effect.

[0029] In addition, it is worthy to be noted that the used in thepresent method is not such that the cell works in extreme conditions.After repetitive experiments, the inventor never found that the presentmethod increased the probability of damage of cell. Hence, the presentmethod is also a safe method.

[0030] Further, the present invention further provides a novel secondarycell or battery having improved specific energy and average operatingvoltage. Contrary to the prior art, the charge cut-off voltage of thesingle cell of said secondary cell or battery is greater than 4.2 V butless than 5.8 V, and the ratio of positive electrode material tonegative electrode material calculated by the theoretical capacity underthe charge cut-off voltage of 4.2 V is 1:1.0 to 1:2.5. As compared tothe prior secondary lithium ion cell or battery cell, the capacity,energy density and output voltage of the present secondary lithium ioncell or battery are greatly improved, while its recycle life isequivalent to that of the prior art cell. For achieving better effect,the aforesaid charge cut-off voltage used in the present invention ispreferably in the range from 4.3 V to 5.2 V, more preferably in therange from 4.3 V to 4.8 V. In addition, the ratio of positive electrodematerial to negative electrode material of said singe lithium ion cell,which is calculated by the theoretical capacity under the charge cut-offvoltage of 4.2 V, is 1:1.5 to 1:2.5. Experiments showed that when theprocess parameters go beyond the aforesaid ranges, the properties ofcell are deteriorated and unsuitable for use. When the ratio of positiveelectrode material to negative electrode material is less than 1.0calculated by theoretical capacity, the recycle life of cell isexcessively reduced, while when said ratio is greater than 2.5, thevolume efficiency of cell is notably reduced. Further, when the chargecut-off voltage is greater than 5.8 V, the cell has inferior propertiesand is unsuitable for use.

[0031] Moreover, it is important that the present method not only can beused to the secondary lithium ion cell or battery of the presentinvention, but also can be used to the secondary lithium ion cell orbattery prepared according to the method of the prior art, such as thepresent commercialized secondary lithium ion cell or battery.

[0032] Without limitation, the following contents more concretelyintroduce the secondary lithium ion cell used in the present method.Generally, a secondary lithium ion cell comprises a positive electrode,a negative electrode, a non-aqueous electrolyte, and a separator thepositive electrode and the negative electrode. The non-aqueouselectrolyte can be obtained by dissolving lithium-containing metal salt,such as LiPF₆, as electrolyte into a non-aqueous solvent, such asethylene carbonate or dimethyl carbonate. The separator can be insolublein said non-aqueous solvent, and is a porous membrane made ofpolyethylene or polypropylene resin. The ratio of positive electrodematerial to negative electrode material is calculated by theoreticalcapacity under the charge cut-off voltage of 4.2 V.

[0033] Positive Electrode

[0034] The positive electrode is prepared e,g., by dispersing positiveelectrode active material, conducting agent and binder in a suitablesolvent to form a suspension, coating said suspension on a currentcollector, such as aluminum foil, then drying and pressing the coatedcurrent collector by rollers.

[0035] The positive electrode active substance used in the presentinvention is lithium-containing compound. Although the examples uselithium cobalt oxides (lithium cobalt composite oxides), lithiummanganese oxides and lithium nickel oxides as positive electrodematerial, it is understood that the practice of the present invention isnot limited to the specific properties of said lithium-containingcomposite oxides, rather a wide range of positive electrode activesubstance can be used in the present invention. The common feature ofthese oxides is that their specific energy increases with the increaseof voltage, and the experiments (see the examples) prove that thecapacity of cell is greatly elevated when the charge cut-off voltage isabove 4.20 V, while the other properties of cell are not affected. Thepresent invention can also be used to lithium ion cells having dopedlithium-containing compound as positive electrode active material, suchas various positive electrode active materials containing various oxidesand sulfides, such as lithium cobalt composite oxides, lithium manganesecomposite oxides, lithium nickel composite oxides, lithium nickel cobaltcomposite oxides, lithium manganese cobalt composite oxides, andvanadium oxides. Among these positive electrode materials, lithiumcobalt composite oxides (such as LiCoO₂), lithium manganese compositeoxides (such as LiMn₂O₄), lithium nickel composite oxides (such asLiNiO₂), lithium nickel cobalt composite oxides (such asLiNi_(1-x)Co_(x)O₂), and lithium manganese cobalt composite oxides (suchas LiMn_(x)Co_(1-x)O₂), which have higher cell voltage, are preferablyused. In addition, the present invention can use conventional conductingagent and binder, and the mixture ratio for each components in thepositive electrode active material can be those well known in the art.

[0036] Separator

[0037] The separator used in the present invention is a separator wellknown in the art. For example, it can be a non-woven fabric made ofsynthetic resin, polyethylene porous membrane or polypropylene porousmembrane, and a material formed by like materials.

[0038] Negative Electrode

[0039] The negative electrode is prepared e.g., by dispersing negativeelectrode active material, conducting agent and binder in a suitablesolvent to form a suspension, coating said suspension on a currentcollector, such as copper foil, nickel foil or stainless steel foil,then drying and pressing the coated current collector by rollers.

[0040] The negative electrode active material used in the presentinvention is a carbonaceous or non-carbonaceous substance capable ofdoping and dedoping lithium ion, including, such as, lithium alloy (suchas Li₄Ti₅O₁₂), metal oxide (such as amorphous tin oxide, WO₂ and MoO₂),TiS₂ and carbonaceous substance capable of absorbing and desorbinglithium ions, and especially, the carbonaceous substance is the desirednegative electrode active material.

[0041] The carbonaceous substance used in the present inventionincludes: graphite, non-oriented graphite, coke, carbon fiber, sphericalcarbon, carbon sintered from resin, carbon grown in gas phase, andcarbon nanometer tube. Since the negative electrode comprising theaforesaid specific carbon fiber or spherical carbon exhibits high chargeefficiency, mesophase asphalt based carbon fiber or mesophase asphaltbased spherical carbon is preferably used as the carbonaceous substance.The mesophase asphalt based carbon fiber or mesophase asphalt basedspherical carbon can be obtained according to the conventional method.

[0042] Without any specific restriction, non-aqueous electrolytes andshells of the type well known in the art can be used in the presentinvention. For example, use can be made of non-aqueous electrolyte is aliquid non-aqueous electrolyte prepared by dissolving electrolyte in anon-aqueous solvent, a gel non-aqueous electrolyte prepared by mixingpolymer, non-aqueous solvent and solute, or a solid polymer non-aqueouselectrolyte, etc.

[0043] The cell structure can be a helical structure formed by wrappinganode and cathode with a separator in between, or a laminate structureformed by stacking anode and cathode with a separator in between, butthe structure of cell is not limited to a certain shape, and it can becylinder shape, prism shape, coin shape, button shape and so on.

[0044] Except voltage, all process parameters for charge and dischargeare commonly used in the art.

[0045] Since the voltage used in the present invention is higher thanthe voltage currently used in the art, the necessary modification ismade to the protecting circuit or relevant devices and equipmentscomprising said circuit which are used in the present method and for thepresent secondary lithium ion cell. The type and structure of theprotecting circuit for said secondary lithium ion cell or battery shallnot be specifically restricted, i.e., the well known structure can beused, provided that the first overcharging protection voltage is greaterthan 4.35 V, and the overcharge release voltage is greater than 4.15 V,preferably the first overcharging protection voltage is greater than4.45 V, and the overcharge release voltage is greater than 4.25 V.

[0046] In addition, the electronic device of the present invention usinga secondary lithium ion cell or battery as energy source shall not bespecifically limited. The device can be a common structure, providedthat said electronic device containing a single lithium ion cell havinga first overcharging protection voltage of greater than 4.35 V, and anovercharge release voltage of greater than 4.15 V, preferably a firstovercharging protection voltage of greater than 4.45 V, and theovercharge release voltage of greater than 4.25 V. The electronic deviceof the present invention includes: notebook type computer, PDA,electromotive bicycle, and electromotive automobile etc.

[0047] Similarly, the charging device for the secondary lithium ion cellor battery of the present invention can be a charging device having anytype and structure with the proviso that said charging device controlsthe charge cut-off voltage of the single lithium ion cell within a rangefrom 4.3 V to 5.8 V, preferably from 4.3 V to 5.2 V, more preferablyfrom 4.3 V to 4.8 V.

EXAMPLES

[0048] The present invention is described in detail hereinafteraccording to the results of specific experiments.

[0049] Preparation of Single Lithium Ion Cell

[0050] In one of examples of the present invention, the process forpreparing the single lithium ion cell is described as follows.

[0051] Said single lithium ion cell uses copper foil as currentcollector of negative electrode, aluminum foil as current collector ofpositive electrode, lithium cobalt oxides as positive electrode activesubstance, and MCMB as negative electrode active substance. The type ofcell is prismatic 653466. The lithium cobalt oxides is mixed with 7%PVDF as binder and 5% conductive carbon black, and then is added intoNMP solvent in a ratio of 1:1. The negative electrode material isdirectly mixed with 10% PVDF as binder in a ratio of 1:1 to form aslurry. The positive electrode slurry is coated on the positiveelectrode current collector, and the negative electrode slurry is coatedon the negative electrode current collector, and then they are dried andpressed, and treated by coating. The coating slurry for the positiveelectrode is prepared by dissolving a mixture of carbonaceous materialand PVDF in an organic solvent, which is coated on the dried positiveelectrode. The coated positive electrode is pressed and baked again.

[0052] The dried positive electrode and negative electrode are connectedto a lead, and a separator made of PP is inserted between them. Afterbeing wrapped by a winder to form an assembly, and this assembly ismounted into a cell shell made of aluminum or steel material. The shelland the cover are soldered together by laser soldering. An electrolytesolution is injected into the cell under relative humidity less than1.5%, wherein said electrolyte solution contains a mixture solvent ofEC:DEC:DMC=1:1:1, and an electrolyte of 1M LiPF₆. The cell isimmediately sealed after the injection.

Example 1

[0053] Several lithium ion cells of quadrate type 653466 are preparedaccording to the above method, and they each have a ratio of positiveelectrode material to negative electrode material of 1:1.0, 1:1.05,1:1.1, 1:1.15,1:1.2, 1:1.25 and 1:1.3. After formation and test using ancharge cut-off voltage of 4.20 V, these cells have an arithmetic meancapacity of 1113 mAh, a weight specific energy of 102 Wh/kg, an averageoperating voltage of 3.70 V, and maintain 85.72% of capacity after 400recycles.

Example 2

[0054] Several lithium ion cells of quadrate type 653466 are preparedaccording to the above method, and they each have a ratio of positiveelectrode material to negative electrode material of 1:1.0, 1:1.05,1:1.1, 1:1.15, 1:1.2, 1:1.25 and 1:1.3. After formation and test usingan charge cut-off voltage of 4.30 V, these cells have an arithmetic meancapacity of 1206 mAh, a weight specific energy of 113 Wh/kg, an averageoperating voltage of 3.75 V, and maintain 86.31% of capacity after 400recycles.

Example 3

[0055] Several lithium ion cells of quadrate type 653466 are preparedaccording to the above method, and they each have a ratio of positiveelectrode material to negative electrode material of 1:1.0, 1:1.05,1:1.1, 1:1.15, 1:1.2,1:1.25 and 1:1.3. After formation and test using ancharge cut-off voltage of 4.35 V, these cells have an arithmetic meancapacity of 1253 mAh, a weight specific energy of 119 Wh/kg, an averageoperating voltage of 3.8 V, and maintain 84.79% of capacity after 400recycles.

Example 4

[0056] Several lithium ion cells of quadrate type 653466 are preparedaccording to the above method, and they each have a ratio of positiveelectrode material to negative electrode material of 1:1.0, 1:1.05,1:1.1, 1:1.15, 1:1.2, 1:1.25 and 1:1.3. After formation and test usingan charge cutoff voltage of 4.40 V, these cells have an arithmetic meancapacity of 1302 mAh, a weight specific energy of 123 Wh/kg, an averageoperating voltage of 3.80 V, and maintain 83.93% of capacity after 400recycles.

Example 5

[0057] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, and it has a ratio of positive electrode materialto negative electrode material of 1:1.45. After formation and test usingan charge cut-off voltage of 4.45 V, said cell has a capacity of 1365mAh, a weight specific energy of 129 Wh/kg, an average operating voltageof 3.85 V, and maintain 84.56% of capacity after 400 recycles.

Example 6

[0058] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, and it has a ratio of positive electrode materialto negative electrode material of 1:1.60. After formation and test usingan charge cut-off voltage of 4.60 V, said cell has a capacity of 1692mAh, a weight specific energy of 165 Wh/kg, an average operating voltageof 3.9 V, and maintain 85.13% of capacity after 400 recycles.

Example 7

[0059] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, and it has the ratio of positive electrode materialto negative electrode material of 1:1.7. After formation and test usingan charge cut-off voltage of 4.80 V, said cell has a capacity of 1824mAh, a weight specific energy of 178 Wh/kg, an average operating voltageof 3.9 V, and maintain 83.92% of capacity after 400 recycles.

Example 8

[0060] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, and it has the ratio of positive electrode materialto negative electrode material of 1:1.9, wherein the substance A havinghigh decomposition potential is added, or the electrolyte solution Bhaving high decomposition potential is used. After formation and testusing an charge cut-off voltage of 5.00 V, said cell has a capacity of1894 mAh, a weight specific energy of 186 Wh/kg, an average operatingvoltage of 3.93 V, and maintain 81.23% of capacity after 400 recycles.

Example 9

[0061] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, wherein 15% of another lithium-containing compoundC is added into the positive electrode slurry, and the ratio of positiveelectrode material to negative electrode material is 1:2.2. Afterformation and test using an charge cut-off voltage of 5.20 V, said cellhas a capacity of 1962 mAh, a weight specific energy of 194 Wh/kg, anaverage operating voltage of 3.96 V, and maintain 81.19% of capacityafter 400 recycles.

Example 10

[0062] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, wherein 15% of another lithium-containing compoundC is added into the positive electrode slurry, and the ratio of positiveelectrode material to negative electrode material is 1:2.3. Afterformation and test using an charge cutoff voltage of 5.40 V, said cellhas a capacity of 1968 mAh, a weight specific energy of 195 Wh/kg, anaverage operating voltage of 3.96 V, and maintain 81.19% of capacityafter 400 recycles.

Example 11

[0063] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, wherein 15% of another lithium-containing compoundC is added into the positive electrode slurry, and the ratio of positiveelectrode material to negative electrode material is 1:2.4. Afterformation and test using an charge cut-off voltage of 5.60 V, said cellhas a capacity of 1970 mAh, a weight specific energy of 195 Wh/kg, anaverage operating voltage of 3.96 V, and maintain 79.97% of capacityafter 400 recycles.

Example 12

[0064] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, wherein 15% of another lithium-containing compoundC is added into the positive electrode slurry, and the ratio of positiveelectrode material to negative electrode material is 1:2.5. Afterformation and test using an charge cut-off voltage of 5.80 V, said cellhas a capacity of 1972 mAh, a weight specific energy of 195 Wh/kg, anaverage operating voltage of 3.96 V, and maintain 78.82% of capacityafter 400 recycles.

Example 13

[0065] Lithium ion cells of quadrate type 653466 are prepared accordingto the above method, wherein 15% of another lithium-containing compoundC is added into the positive electrode slurry, and the ratio of positiveelectrode material to negative electrode material is 1:2.6. Afterformation and test using an charge cut-off voltage of 5.90 V, said cellhas a capacity of 1565 mAh, and maintain 12.31% of capacity after 15cycles.

[0066] It can be seen from the above examples that when the chargevoltage is elevated above 4.2 V, the capacity, operating voltage andweight specific energy increase with the elevation of the chargevoltage; and when the charge voltage is elevated above 4.45 V, thecapacity, operating voltage and weight specific energy continuouslyfurther increase with the elevation of the charge voltage and of theratio of positive electrode material to negative electrode material; butwhen the charge voltage is greater than 5.8 V, the properties of celldecrease.

Examples 14-18

[0067] Using lithium manganese oxide as positive electrode activematerial. Lithium ion cells are prepared according to the aforesaidmethod, wherein the positive electrode slurry consists of 4 g lithiummanganese oxide, 0.6 g carbon black, 0.32 g PVDF, and 4.92 g NMP, andthe negative electrode slurry consists of 2.6 g MCMB, 0.26 g PVDF, and3.0 g NMP. The cell is a quadrate type 653466 lithium ion cell. The dataof experiments are depicted in Table 2. TABLE 2 Charge cut-off CapacityExample voltage (V) Processing change Recycle property (mAh) 14 4.20 —Maintaining 84.23% 375 after 400 recycles 15 4.30 — Maintaining 85.12%405 after 400 recycles 16 4.60 The ratio of positive electrodeMaintaining 83.45% 575 material to negative electrode after 400 recyclesmaterial is 1:1.6. 17 5.80 The ratio of positive electrode Maintaining82.83% 567 material to negative electrode after 400 recycles material is1:2.5, and substance A is added into the electrolyte solution. 18 5.90The ratio of positive electrode Maintaining 54.23% 312 material tonegative electrode after 30 cycles material is 1:2.7, and substance A isadded into the electrolyte solution.

[0068] Examples 14-18 show that when using lithium manganese oxide aspositive electrode active material, the cells obtain the results similarto that of examples 1-13. Namely, when the charge voltage is elevatedabove 4.2 V, the capacity, operating voltage and weight specific energyincrease with the elevation of the charge voltage; and when the chargevoltage is elevated above 4.45 V, the capacity, operating voltage andweight specific energy continuously increase with the elevation of thecharge voltage and of the ratio of positive electrode material tonegative electrode material; but when the charge voltage is greater than5.8 V, the properties of cell decrease.

Examples 19-23

[0069] Using lithium nickel oxide as positive electrode active material.Secondary lithium ion cells are prepared according to the aforesaidmethod for preparing the lithium ion cells using lithium cobalt oxide aspositive electrode active material, wherein the positive electrodeslurry consists of 4 g lithium nickel oxide, 0.2 g carbon black, 0.2 gPVDF, and 4.80 g NMP, and the negative electrode slurry consists of 3.5g MCMB, 0.35 9 PVDF, and 4.0 g NMP. The cell is a quadrate type 063048lithium ion cell. The data of experiments are depicted in Table 3. TABLE3 Charge cut-off Capacity Example voltage (V) Processing change Recycleproperty (mAh) 19 4.20 — Maintaining 610 81.23% after 400 recycles 204.30 — Maintaining 675 80.72% after 400 recycles 21 4.60 The ratio ofpositive Maintaining 912 electrode material to 78.32% after 400 negativeelectrode material recycles is 1:1.6. 22 5.80 The ratio of positiveMaintaining 879 electrode material to 75.83% after 400 negativeelectrode material recycles is 1:2.5, and substance A is added into theelectrolyte solution. 23 5.90 The ratio of positive Maintaining 796electrode material to 47.53% after 50 negative electrode material cyclesis 1:2.7, and substance A is added into the electrolyte solution.

[0070] Examples 19-23 show that when using lithium nickel oxide aspositive electrode active material, the cells obtain the results similarto those of examples 1-16. Namely, when the charge voltage is elevatedabove 4.2 V, the capacity, operating voltage and weight specific energyincrease with the elevation of the charge voltage; and when the chargevoltage is elevated above 4,45 V, the capacity, operating voltage andweight specific energy continuously increase with the elevation of thecharge voltage and of the ratio of positive electrode material tonegative electrode material; but when the charge voltage is greater than5.8 V, the properties of cell decrease.

Examples 24-28

[0071] Using cobalt-doping lithium nickel oxide as positive electrodeactive material, a secondary lithium ion cell is prepared according tothe aforesaid method for preparing the secondary lithium ion cells usinglithium cobalt oxide as positive electrode active material, wherein thepositive electrode slurry consists of 4 g lithium nickel oxide, 0.2 gcarbon black, 0.2 g PVDF, and 4.80 g NMP, and the negative electrodeslurry consists of 3.5 9 MCMB, 0.35 g PVDF, and 4.0 g NMP. The cell is aquadrate type 063048 lithium ion cell. The data of experiments aredepicted in Table 4. TABLE 4 Charge cut-off Capacity Example voltage (V)Processing change Recycle property (mAh) 24 4.20 — Maintaining 62381.23% after 400 recycles 25 4.30 — Maintaining 682 80.72% after 400recycles 26 4.60 The ratio of positive Maintaining 934 electrodematerial to 78.32% after 400 negative electrode material recycles is1:1.6. 27 5.80 The ratio of positive Maintaining 919 electrode materialto 75.83% after 400 negative electrode material recycles is 1:2.5, andsubstance A is added into the electrolyte solution. 28 5.90 The ratio ofpositive Maintaining 697 electrode material to 39.53% after 50 negativeelectrode material cycles is 1:2.7, and substance A is added into theelectrolyte solution.

[0072] Examples 24-28 show that when using cobalt-doping lithium nickeloxide as positive electrode active material, the cells obtain theresults similar to those of examples 1-21. Namely, when the chargevoltage is elevated above 4.2 V, the capacity, operating voltage andweight specific energy increase with the elevation of the chargevoltage; and when the charge voltage is elevated above 4.45 V, thecapacity, operating voltage and weight specific energy continuouslyincrease with the elevation of the charge voltage and of the ratio ofpositive electrode material to negative electrode material; but when thecharge voltage is greater than 5.8 V, the properties of cell decrease.In the meantime, these results indicate that the present method is notlimited to some specific lithium-containing composite oxides as positiveelectrode material, but can be widely used in various secondary lithiumion cells.

[0073] The present method can be used in the production of lithium ioncell and for improvement of relevant products in the following aspects.

[0074] 1. After adjustment of ratio of positive electrode material tonegative electrode material so that the single lithium ion cell obtainsthe optimal capacity calculated by theoretical specific energy, asecondary lithium ion cell having high capacity and excellent recycleproperty can be obtained by controlling the charge cut-off voltagewithin a range from 4.2 V to 5.8 V during the formation and test of thecell. The merits of this method lie in that the specific energy andaverage operating voltage are greatly improved without substantiallyincreasing the cost and simultaneously with the effects that theoriginal formation process should achieve. Thus, as to the manufacturerof lithium ion cell, the profit margin and the competitiveness ofenterprise are improved: and as to the products using lithium ion cells,their performances are improved with the increase of capacity andaverage operating voltage of the lithium ion cells.

[0075] 2. The protecting circuit of single secondary lithium ion cellbased on the present invention possesses an overcharge release voltageof greater than 4.15 V, and an overcharge first protection voltage ofgreater than 4.35 V.

[0076] 3. The single lithium ion cell prepared according to the presentinvention can combine with the aforesaid protecting circuit having thetechnical features of the present invention and form a secondary lithiumion cell or battery.

[0077] 4. The protecting circuit having the technical features of thepresent invention can be used in a mobile electronic device and producthaving a secondary lithium ion cell or battery as energy source.

[0078] The technology of the present invention possesses a greatcommercial value for improving the capacity, average operating voltageand specific energy of the secondary lithium ion cell, and makes acertain contribution to the research of the basic theory of lithium ioncell. It is believable that the present technology could improve thedevelopment of the whole secondary lithium ion cell industry.

1. A method for improving the capacity, average operating voltage andspecific energy of a secondary lithium ion cell or battery,characterized in that the charge cut-off voltage of the singe cell isgreater than 4.2 V but less than 5.8 V; and the ratio of positiveelectrode material to negative electrode material of the single cell isfrom 1:1.0 to 1:2.5, as calculated by the specific capacity with thecharge voltage limited to 4.2 V.
 2. A method according to claim 1,characterized in that the charge cut-off voltage of the singe cell iswithin a range from 4.3 V to 5.2 V.
 3. A method according to claim 1,characterized in that the charge cut-off voltage of the singe cell iswithin a range from 4.3 V to 4.8 V.
 4. A method according to claim 1,characterized in that the ratio of positive electrode material tonegative electrode material of the single cell is from 1:1.15 to 1:2.5.5. A secondary lithium ion cell or battery, characterized in that thesingle secondary lithium ion cell has a charge cut-off voltage ofgreater than 4.2 V but less than 5.8 V, and the ratio of positiveelectrode material to negative electrode material of the single cell isfrom 1:1.0 to 1:2.5. as calculated by the theoretic capacity with thecharge cut-off voltage set at 4.2 V.
 6. A secondary lithium ion cell orbattery according to claim 5, characterized in that the single secondarylithium ion cell has a charge cut-off voltage within a range from 4.3 Vto 5.2 V.
 7. A secondary lithium ion cell or battery according to claim5, characterized in that the single secondary lithium ion cell has acharge cut-off voltage within a range from 4.3 V to 4.8. V.
 8. Asecondary lithium ion cell or battery according to claim 5,characterized in that the ratio of positive electrode material tonegative electrode material of the single cell is from 1:1.15 to 1:2.5.9. A secondary lithium ion cell or battery according to claim 5,characterized in that the single lithium ion cell has a firstovercharging protection voltage of greater than 4.35 V, and anovercharging protection release voltage of greater than 4.15 V.
 10. Asecondary lithium ion cell or battery according to claim 9,characterized in that the single lithium ion cell has a firstovercharging protection voltage of greater than 4.45 V, and anovercharge protection release voltage of greater than 4.25 V.
 11. Aprotecting circuit for the secondary lithium ion cell or batteryaccording to claim 5, characterized in that the single lithium ion cellof said protecting circuit has a first overcharge protection voltage ofgreater than 4.35 V, and an overcharging protection release voltage ofgreater than 4.15 V.
 12. A protecting circuit according to claim 11,characterized in that the single lithium ion cell of said protectingcircuit has a first overcharge protection voltage of greater than 4.45V, and an overcharge protection release voltage of greater than 4.25 V.13. An electronic device using a secondary lithium ion cell or batteryas energy source, characterized in that said electronic device comprisesa protecting circuit, wherein the single lithium ion cell has a firstovercharging protection voltage of greater than 4.35 V, and anovercharge protection release voltage of greater than 4.15 V.
 14. Anelectronic device according to claim 13, characterized in that saidelectronic device comprises a protecting circuit, wherein the singlelithium ion cell has a first overcharging protection voltage of greaterthan 4.45 V, and an overcharge protection release voltage of greaterthan 4.25 V.
 15. A charging device for a secondary lithium ion cell orbattery, characterized in that said charging device controls a chargecut-off voltage to the single lithium ion cell within the range ofgreater than 4.3 V but less than 5.8 V.
 16. A charging device accordingto claim 15, characterized in that said charging device controls acharge cut-off voltage to the single lithium ion cell within the rangefrom 4.3 V to 5.2 V.
 17. A charging device according to claim 15,characterized in that said charging device controls a charge cut-offvoltage to the single lithium ion cell within the range from 4.3 V to4.8 V.